30 WELLS'S NATURAL PHILOSOPHY. 



ice. The contact is thus interrupted, and the adhesion between the rail and 

 Tvheel is impaired. 



9. When a liquid adheres to a solid, what term do vre apply to designate the act of 

 adhesion 1 



"Wetting. It is necessary that a liquid should adhere to the surface of a solid 

 before it can bo wet. Water falling upon an oiled surface does not wet it, 

 because there is no adhesion between the particles of the oil and the partidea 

 of the water. 



10. Why are drops of rain, of tears, and of dew upon tho leaves of plants, generally 

 •pherical, or globular ? 



The force of cohesion always tends to cause the particles of a liquid, when 

 unsupported, or supported on a surface having little attraction for it, to as- 

 sume the form of a sphere — a globe, or sphere, being the figure which will 

 contain the greatest amount of matter within a given surface. 



This property of fluids is taken advantage of in the arts, in the manufacture 

 of shot. The melted lead is made to fall in a shower, from a great elevation. 

 In its descent the drops become globular, and before they reach the end of 

 their fall become hardened by cooling, and retain their form. 



CHAPTER IV. 



ATTRACTION" OF GRAVITATION. 



57. The Attraction of Gravitation is 

 traedon' of tliat form of attraction, by which all bodies at 

 Graviution? sensible distances, tend to approach each other. 



Electricity and Magnetism attract bodies at sensible dis- 



Gravitation tances also, but their influenca upon difterent classes of bodies 



differ from varies, and is Mmited by distance. Molecular, or Internal At- 

 other forms . ..,,,. mi . • * 



of attraction? traction, acts only at insensible distances. The Attraction of 



Gravitation acts at all distances, and upon all bodies. 



58, Every portion of matter in the universe 



What is the i • • i i> 



great law of attracts cvcry other portion, with a lorce pro* 



the attraction • i i- i • • -i 



of Gravita- portioncd dircctly to its mass, or quantity, and 

 inversely as the square of the distance. Thia 

 is the great general law of the Attraction of Gravitation. 



By the Attraction of Gravitation being directly proportional to the mass of 

 a body, we mean, that if of two bodies, the mass of one be twice as large as 

 tliat of the other, its force of attraction will be twice as great : if it is only 

 half as large, its attraction will be only half as great. 



By the Attraction of Gravitation being inversely proportioned to the square 



